An IO signal can be received by a receiver that detects the IO signal and amplifies it. The IO signal is then input to a core circuit for further processing. Since the IO signal is not generated by the core circuit, its amplitude is not limited by the core circuit's power supply voltage (also known as the core operation voltage or Vdd). The amplitude of the IO signal may be as high as the IO circuit's power supply voltage (also known as the IO operation voltage or Vddio). Therefore, the core circuit that directly takes input from an IO signal receiver must be able to handle an IO signal with amplitude of Vddio.
In a traditional circuit that employs micron technology or earlier technologies, the operation voltage of the core circuit was in a range of about 2.5V to 3.3V, similar to the IO operation voltage of 3.3V to 5.0V, so that amplified IO signal can be directly input to a core circuit since their operation voltages are in same range. However, with the integrated circuit size continuing to shrink, the core operation voltage is lowered. For example, a circuit made with sub micron technology (0.5 to 0.8 micron) has an operation voltage of about 2.5V to about 3.3V. When the deep sub micron technology is employed, the size of a circuit is further reduced to about 0.25, 0.18 or even 0.13 micron, and the operation voltage drops to around 1 volt. It is expected that the core operation voltage will continue to fall with the integrated circuit size continuing to shrink.
While the core circuit operation voltage falls, the IO circuit voltage often stays at a higher voltage. Problems can arise when a high IO signal with the amplitude Vddio is input to a core circuit. The core circuit can be damaged or degraded if it is only designed to handle a voltage no higher than its operation voltage Vdd. Traditionally, a differential amplifier can be used as the IO signal receiver due to the compatibility of Vddio and Vdd.
FIG. 1, which was taken from U.S. Pat. No. 4,958,133, illustrates a circuit schematic view of a conventional IO receiver. A reference voltage is applied at node A and an IO signal voltage is applied at node B. The IO signal is amplified and output to node OUT. It is to be noted that the power supply at node C is the same as the power supplies at nodes D and E. This circuit works fine in the micron technology. However, in deep micron technology, since the power voltage at node C is around 1V, while the input IO signal at node B is 2.5V to 3.3V, the circuit does not function correctly. The input IO signals are clamped by transistors 1b and 2b when the input signal is higher than Vdd. Transistors 1b and 2b and surrounding transistors may be damaged or degraded by voltages higher than they are designed to handle.
A solution to this problem is to use a device called level-down converter. This device takes an input signal with the amplitude of the IO operation voltage (Vddio) and converts to a relatively low signal with the amplitude of the core operation voltage (Vdd). FIG. 2 is a schematic view of how a traditional circuit 50 works. The signal Vin is an IO signal input. Vref is a reference voltage used to decide whether the input signal is a “1” or “0”, and is normally a fixed voltage. The receiver circuit 52 is used to receive the input IO signal Vin and amplify it. After amplification, the amplitude of the IO signal is Vddio. A level-down converter 54 is used to reduce its amplitude to Vdd before the signal is sent to a core circuit 56. Level-down converter 54 is normally a stand-alone device.
This solution has an intermediate stage at which the IO signal is amplified to a high level of Vddio. This not only increases power consumption but also degrades the circuit response to high-speed IO signals. The response degradation was observed when the signal speed reaches several hundred mega hertz. Two factors may have contributed to the degradation. Firstly, charging a device to a higher voltage and discharging it requires more time, but the high-speed signals have only a short time to change state, so that the signals may be distorted. Secondly, an IO signal is a noise source. When a signal changes state, electromagnetic noise that interferes with other parts of the circuit are generated. A higher voltage will generate a higher amount of noise. For example, a 3.3V signal generates much higher noise than a 1.2V signal.